Examples of electrochromic systems are described in Patents U.S. Pat. No. 5,239,406 and EP-612 826.
Electrochromic systems have been extensively studied. They are known to comprise in general two layers of electrochromic materials separated by an electrolyte and flanked by two electrodes. Each of the electrochromic layers, under the effect of an electrical supply, can inject charges reversibly, the change in their oxidation state as a result of these injections/ejections resulting in a change in their optical and/or thermal properties (for example, in the case of tungsten oxide, a switch from a blue coloration to a colourless appearance).
It is common practice to classify electrochromic systems in three categories:                that in which the electrolyte is in the form of a polymer or a gel, for example, a protonically conducting polymer such as those described in Patents EP-253 713 or EP-670 346, or a polymer conducting by lithium ions such as those described in Patents EP-382 623, EP-518 754 and EP-532 408, the other layers of the system generally being of an inorganic nature;        that in which the electrolyte is an essentially inorganic layer. This category is often referred to by the term “all-solid-state” system—examples of such may be found in Patents EP-867 752 and EP-831 360, French Patent Application FR-2 791 147 and French Patent Application FR-2 781 084; and that in which all the layers are based on polymers, the category then being often referred to by the term “all-polymer” system.        
Many applications have already been envisaged for these systems. They are employed most generally as glazing for buildings or as glazing for vehicles, especially as sunroofs, or else, when they operate in reflection and no longer in transmission, as antidazzle rearview mirrors.
However, for all these applications, owing to their sensitivity to environmental attack, the electrically controllable devices are not generally inserted as they are within a substrate, but are protected within a laminated substrate that also incorporates at least one lamination interlayer, which is generally a polymer film.
This polymer film may optionally have a solar-protection function for the purpose of protecting the layers of the electrochromic system from being heated up by infrared radiation. In another configuration, the solar-protection function is attached to the layers of the stack of the electrochromic system.
However, and whatever the origin of the solar-protection function, the reliability of the electrochromic systems, which may for example be expressed in terms of the durability of the switching of cycles between a colouration state and a non-colouration state of the active layers, is dependent on the temperature, and any temperature variation (in particular an increase in temperature of the active layers as a result of intensive exposure to infrared radiation (electrochromic roof placed in full sunlight)) results in an increase in the rate of degradation of the said active layers.
Conventionally, this solar-protection function is performed by an antireflection coating, which usually consists of a stack of thin interferential layers, in general an alternation of layers based on a dielectric material or based on a metal oxide, especially of the metal oxide, nitride or oxynitride type, having high and low refractive indices.
Although it perfectly fulfils its role of protecting the active layers of the electrochromic system from infrared radiation, the solar-protection layer, which is optionally incorporated within the lamination interlayer, must also withstand the mechanical stresses resulting from the lamination.
These stresses are all the more substantial when the laminated substrate has a complex profile (a substrate with a large deflexion in at least one direction, such as a doubly curved substrate).